1. Field of the Invention
The present invention relates to a fuel injector used in, for example, a gas turbine engine and including a combined fuel injector configured by combining a plurality of fuel nozzles, and particularly to a main injector of the fuel injector.
2. Description of the Related Art
In recent years, in consideration of the environment, there is a need for a reduction of NOx (nitrogen oxide) emitted from gas turbine engines. The NOx to be emitted from the gas turbine engine is generated mainly by oxidization of nitrogen in inflow air when fuel is supplied to the inflow air and combusted at high temperature. Meanwhile, the amount of CO2 emission of the gas turbine engine, that is, fuel consumption decreases as an exhaust gas at an exit of a combustor increases in temperature. Therefore, to reduce the CO2, the fuel needs to be combusted at high temperature by increasing a fuel-air ratio. According to a fuel nozzle of a combustor of a conventional gas turbine engine, the fuel is directly sprayed to a combustion chamber without premixing the fuel with the air. Therefore, before the fuel is adequately mixed with the air, the fuel combusts, and regions where a flame temperature is significantly higher than an average value are generated locally. The amount of NOx generation increases exponentially with the flame temperature. Therefore, a large amount of NOx is generated from the local regions where the flame temperature is high. On this account, according to the conventional combustion method, when the temperature of the exhaust gas at the exit of the combustor is increased, the amount of NOx emission increases sharply.
To reduce the local regions where the flame temperature is high, a lean premix combustion method is effective. According to this method, the fuel and the air are premixed, and a fuel-air mixture in which the fuel in the form of a mist is dispersed in the air is supplied to the combustion chamber and combusted therein. Meanwhile, according to the lean premix combustion method, in a case where the output of the gas turbine engine is low and the fuel-air ratio is low, the flame is unstable and incomplete combustion tends to occur as compared to a case where the fuel is directly sprayed to the combustion chamber. Here, a concentric fuel injector has been devised. This fuel injector is configured such that a pilot injector and a main injector provided outside the pilot injector are provided coaxially. When the output of the gas turbine engine is low, the fuel is directly sprayed from only the pilot injector to the combustion chamber to maintain stable combustion. When the output of the gas turbine engine is intermediate or high, that is, when the amount of NOx emission is large, the amount of fuel injected directly from the pilot injector is reduced, and a pre-mixture generated by the main injector is also injected to the combustion chamber. With this, the amount of NOx emission is reduced. Regarding a gas turbine engine for aircrafts, the output of the gas turbine engine is substantially low (lower than about 40% of the rated output) in a state of each of ground idle, flight idle, and approach, the output of the gas turbine engine is substantially intermediate (about 40 to 80% of the rated output) in a cruising state, and the output of the gas turbine engine is substantially high (about 80 to 100% of the rated output) in a state of each of climb and takeoff.
According to the concentric fuel injector, the temperature of the gas at an entrance of the combustor and the fuel-air ratio when the output of the gas turbine engine is intermediate are respectively lower than those when the output of the gas turbine engine is high, and the flame temperature when a main pre-mixture is combusted is low. Therefore, although the amount of NOx emission is generally not so large, the main pre-mixture tends to cause the incomplete combustion, and the combustion efficiency tends to be low. On this account, when the output of the gas turbine engine is intermediate, flame holding of the main pre-mixture by combustion flame of the pilot injector becomes important. Meanwhile, when the output of the gas turbine engine is high, the flame temperature is adequately high, so that the flame stabilizes only by the main pre-mixture, and the combustion efficiency has no problem. However, the amount of NOx emission tends to be large. Therefore, uniformization of the main pre-mixture needs to be further considered. To maintain satisfactory performance when the output of the gas turbine engine is intermediate or high, a fuel injector has been proposed, in which: two air channels through which air inflows in a radial direction are formed on the main injector; and fuel injection holes corresponding to one fuel supply system are formed on each air channel, that is, the fuel injection holes corresponding to each of two fuel supply systems are formed (see Japanese Laid-Open Patent Application Publication No. 2003-262337). According to this fuel injector, when the output of the gas turbine engine is intermediate, the fuel is injected only to one air channel through which the pre-mixture is supplied to a position closer to the pilot injector. With this, the flame holding of the pre-mixture by the combustion flame of the pilot injector is promoted, and this improves the combustion efficiency. Meanwhile, when the output of the gas turbine engine is high, the fuel is injected to both air channels, and the pre-mixture is generated more uniformly. Thus, the NOx emission is suppressed.
According to the fuel injector of Japanese Laid-Open Patent Application Publication No. 2003-262337, since the fuel is injected to the two air channels, the fuel injection holes corresponding to each of the two fuel supply systems are formed. Forming the fuel injection holes corresponding to each of a plurality of fuel supply systems as above increases structural complexity of the fuel injector. Generally, according to the concentric fuel injector, when the output of the gas turbine engine is low, the fuel flows only to the pilot injector, and the fuel in a fuel channel in the main injector stays therein. The staying fuel causes carbonization (coking) at a certain temperature or higher and accumulates on a wall surface of the fuel channel to close the fuel channel. As a countermeasure against this problem, in the main injector, a cooling structure for preventing the coking needs to be provided for each of the fuel supply systems of the fuel injection holes. However, providing the cooling structures respectively for the plurality of fuel supply systems of the fuel injection holes within a limited space in the main injector extremely increases the structural complexity of the main injector. Moreover, in a case where the fuel injection holes to be used are switched in accordance with the output, a control mechanism is required, and it is extremely difficult to secure reliability for ensuring appropriate selection of the fuel injection holes in any scene.